Chapter 18
The Cardiovascular System: The Heart
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18
ventricles. Each SL valve is fashioned from three pocketlike cusps,
each shaped roughly like a crescent moon (
semilunar
5
half-moon).
Like the AV valves, the SL valves open and close in response
to differences in pressure. When the ventricles contract and
intraventricular pressure rises above the pressure in the aorta
and pulmonary trunk, the SL valves are forced open and
their cusps flatten against the arterial walls as blood rushes
past them
(Figure 18.8a)
. When the ventricles relax, and the
blood (no longer propelled forward by ventricular contrac-
tion) flows backward toward the heart, it fills the cusps and
closes the valves (Figure 18.8b).
We complete the valve story by noting what seems to be an im-
portant omission—there are no valves guarding the entrances of
the venae cavae and pulmonary veins into the right and le± atria,
respectively. Small amounts of blood
do
spurt back into these ves-
sels during atrial contraction, but backflow is minimal because of
the inertia of the blood and because as it contracts, the atrial myo-
cardium compresses (and collapses) these venous entry points.
Homeostatic Imbalance
18.2
Heart valves are simple devices, and the heart—like any me-
chanical pump—can function with “leaky” valves as long as the
impairment is not too great. However, severe valve deformities
can seriously hamper cardiac function.
Attached to each AV valve flap are tiny white collagen cords
called
chordae tendineae
(kor
9
de ten
0
9
ne-e; “tendinous
cords”), “heart strings” which anchor the cusps to the papillary
muscles protruding from the ventricular walls (Figure 18.6c, d).
When the heart is completely relaxed, the AV valve flaps hang
limply into the ventricular chambers below and blood flows into
the atria and then through the open AV valves into the ventri-
cles
(Figure 18.7a)
. When the ventricles contract, compressing
the blood in their chambers, the intraventricular pressure rises,
forcing the blood superiorly against the valve flaps. As a result,
the flap edges meet, closing the valve (Figure 18.7b).
Te chordae tendineae and the papillary muscles serve as
guy-wires that anchor the valve flaps in their
closed
position.
If the cusps were not anchored, they would be blown upward
(everted) into the atria, in the same way an umbrella is blown
inside out by a gusty wind. Te papillary muscles contract with
the other ventricular musculature so that they take up the slack
on the chordae tendineae as the full force of ventricular contrac-
tion hurls the blood against the AV valve flaps.
Semilunar (SL) Valves
Te
aortic
and
pulmonary
(semilunar, SL) valves
guard the bases
of the large arteries issuing from the ventricles (aorta and pulmo-
nary trunk, respectively) and prevent backflow into the associated
1
Blood returning to the heart
fills atria, pressing against the
AV valves. The increased
pressure forces AV valves open.
1
Ventricles contract, forcing
blood against AV valve cusps.
2
As ventricles fill, AV valve flaps
hang limply into ventricles.
2
AV valves close.
3
Atria contract, forcing additional
blood into ventricles.
3
Papillary muscles contract
and chordae tendineae tighten,
preventing valve flaps from
everting into atria.
(a) AV valves open; atrial pressure greater than ventricular pressure
(b) AV valves closed; atrial pressure less than ventricular pressure
Direction of
blood flow
Atrium
Ventricle
Cusp of
atrioventricular
valve (open)
Chordae
tendineae
Papillary
muscle
Atrium
Blood in
ventricle
Cusps of
atrioventricular
valve (closed)
Figure 18.7
The atrioventricular (AV) valves.
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